# Chemical Reactions and Equations

Chemical reactions
The process by which the atoms of 1+ substances are readranged to form diff substances (process always involves a chemical change and sometimes a physical change)
Indicators of a Chemical Reaction
Production of a gas, odor, or precipitate (solid), change in temperature or color
Word equations
Use words or names to represent chemical reactions
Skeleton equations
Use chemical formulas to represent chemical reactions
Chemical equations
Coefficients are added to the skeleton equation in order to satisfy the Law of Conservation of Mass
The Law of Conservation of Mass
Mass is neither created nor destroyed during a chemical reaction but is destroyed
Chemical equations are statements used to describe a chemical reaction by showing:
The reactants & products; the relative amounts of each
Reactants
Starting substances (left side of equation)
Products
Substance(s) formed in the reaction (right side of equation)
Subscript
Number to the right of an element that indicates the number of atoms present in a compound
Coefficients
Numbers to the left of a formula that are used to balance a chemical reaction
State of Matter
(S) = solid,
(L) = liquid,
(G) = gas,
(Aq) = aqueous / water solution
Yields sign
An arrow separating the reactants and the products
To indicate if a catalyst is required
Write the catalyst above the yields sign (a catalyst increases the rate of a chemical reaction without itself undergoing any permanent chemical change)
To indicate that heat is required
Write a triangle above the yields sign
To indicate a particular temperature is required
Write the temperature above the yields sign
Can you change subscripts when balancing a chemical equation?
No!
To balance an equation, you must…
Find the correct coefficients for the chemical formulas in a skeleton equation. Coefficients are whole numbers and should be written as the lowest whole # ratio.
Balancing by inspection
Atoms of each element are counted and balanced, trial and error process, best for simple chemical formulas
Balancing using linear algebra
Systematic approach using a series of linear equations, best for complicated formulas & reactions
Steps for balancing by inspection
1.) write the skeleton equation using correct formulas
2.) count the # of atoms of each element for BOTH sides of the equation & list below
3.) insert coefficients for atoms of one element at a time so that the law of conservation of mass is satisfied
4.) check to see if coefficients are in the lowest possible ratio
Diatomic elements
Br I N Cl H O F
Chemists classify reactions into five types:
1.) synthesis
2.) decomposition
3.) single replacement
4.) double replacement
5.) combustion
Synthesis Reactions
A chemical reaction in which 2+ substances react to produce a single product
A + B -> AB
Decomposition Reactions
A single compound breaks down into 2+ elements or new compounds
AB + energy -> A + B
Decomposition reactions often require an energy source, such as heat, light, or electricity to occur. If energy is required it’s endothermic
Single Replacements Reactions
A reaction in which the atoms of one element replace the atoms of another element in a compound.
A + BX -> AX + B
Like replaces like
Metal elements will replace metal; nonmetal elements will replace nonmetal
What does the reactivity of an element determine?
The reactivity of an element determined if that element will replace another. An element cannot replace a more reactive element
Double Replacement Reactions
Reactions involving the exchange of ions between two compounds
AX + BY -> AY + BX
A & B represent the cations, X & Y represent the anions. The charges of the ions will not change. Reactions will always take place in water.
Neutralizing Reaction
A type of double replacement reaction between an acid and a base
Product will always be a salt (ionic compound) and water
Acid
A molecular compound that ionizes to produce H+
Base
An ionic compound that contains -OH
Combustion Reactions
Oxygen combined with a substance and releases energy in the form of heat & light
O2 is always a reactant
CO2 and H2O are always products
Energy is always released
Formula for combustion reactions
Hydrocarbon + O2 -> CO2 + H2O + energy
Examples of hydrocarbons
CH4 (methane)
C2H6O (ethanol)
C3H8 (propane)
C4H10 (butane)
C8H18 (octane)